Journal of Shellfish Research, Vol. 30, No. 3, 583–608, 2011. NATURAL AND ANTHROPOGENIC FORCES SHAPE THE POPULATION GENETICS AND RECENT EVOLUTIONARY HISTORY OF EASTERN UNITED STATES BAY SCALLOPS (ARGOPECTEN IRRADIANS ) THERESA M. BERT,* WILLIAM S. ARNOLD,† ANNE L. MCMILLEN-JACKSON, AMI E. WILBUR‡ AND CHARLES CRAWFORD Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, 100 Eighth Avenue Southeast, St. Petersburg, Florida 33701 ABSTRACT Bay scallops (Argopecten irradians Lamarck) are ecologically important in U.S. Atlantic waters off northeastern states and in the Florida Gulf of Mexico, and have been intensely harvested from both of those regions for decades. However, a detailed study comparing their basic population genetic structures using more than a single type of genetic marker has not been conducted. Through such a study, key phylogeographic, taxonomic, and fisheries issues can be addressed. We used variation in allozyme loci and mitochondrial DNA restriction fragment length polymorphisms to evaluate and compare the population genetic structures of bay scallops from those two regions, to propose a new interpretation for the composition of the North Carolina bay scallop population, to resolve the taxonomic quandary of Argopecten irradians taylorae, and to evaluate the apparent and potential genetic effects of the common fishery practice of hatchery-based stock enhancement on the genetic diversity and relatedness of Atlantic bay scallop populations. Atlantic Ocean (North Carolina through New York) bay scallop populations are genetically more distant from each other than are Florida Gulf bay scallop populations, except those in Florida Bay. Each Atlantic population has a different phylogeographic history, is quasi-independent, and should be treated as a genetically unique entity. The North Carolina bay scallop population is composed of Argopecten irradians irradians individuals, but also has genetic input from Argopecten irradians concentricus. Bay scallops occurring in Florida Bay constitute a population of A. i. concentricus that has diverged from other Florida Gulf populations because it has undergone repeated contractions and expansions of varying magnitude and is nearly isolated from other bay scallop populations. For the common practice of hatchery-based stock enhancement in the Atlantic, broodstock bay scallops should be taken from the same genetic population, and all stock enhancement efforts should include comprehensive genetic monitoring programs. In some cases, improving the abundance and density of bay scallop aggregations through habitat improvement may be preferable to stock enhancement for bay scallop restoration, but in other cases genetically conscientious stock supplementation or restoration may be the only alternative. KEY WORDS: aquaculture, Argopecten irradians, Atlantic, bay scallop, evolution, fishery, Florida, Gulf of Mexico, population genetics, stock enhancement, taxonomy INTRODUCTION in the extreme southeastern United States and eastern Gulf, and Argopecten irradians amplicostatus occurs in the western Gulf The charismatic, environmentally, and economically impor- (Waller 1969) (Fig. 1A). Since the first publication of the sub- tant bay scallop Argopecten irradians (Lamarck) inhabits shallow- species’ ranges, and despite numerous taxonomic studies that water seagrass flats and algal beds in the eastern United States. In evaluated variation in morphometrics, meristics, and multiple the western North Atlantic Ocean (henceforth, Atlantic), bay types of genes, the locations of sympatry between A. i. irradians scallops range in the United States from Massachusetts (Clarke and A. i. concentricus in the Atlantic, the taxonomic identity of 1965) through North Carolina (NC) and, below a 10° latitudinal A. irradians in NC, and the existence of a fourth subspecies gap extending from South Carolina to east–central Florida (Argopecten irradians taylorae (Petuch 1987)) in and around FB (Heffernan et al. 1988), occur again in southeastern Florida from have remained ambiguous. The confusion is understandable, West Palm Beach to Biscayne Bay (Marelli et al. 1997a) (Fig. 1). given the considerably different results that can emerge among In the Gulf of Mexico (henceforth, Gulf), bay scallops range these character sets, even when all data are taken from the same from Florida Bay (FB) northward through Florida Gulf waters individuals (e.g., Wilbur 1995, Wilbur & Gaffney 1997). and westward to the Chandeleur Islands, LA (Waller 1969). Population genetics analyses have been woven into these Westward of a gap along the northern Gulf, they occur again principally taxonomic studies, and also into other ecological from northeastern Texas southward through Mexico (Waller and biological studies on bay scallops, but a thorough compar- 1969, Wakida-Kusunoki 2009) to Colombia (Abbott 1974). ison of the population genetic structure of bay scallops in the Within its range, A. irradians has been divided into several Atlantic and Gulf basins using multiple genetic markers has not subspecies. Argopecten irradians irradians occurs in the north- yet been conducted. Understanding the population genetic eastern United States, Argopecten irradians concentricus occurs structure of bay scallops is important for conservation and restoration of populations as well as for fisheries management *Corresponding author. E-mail: [email protected] because destructive natural and anthropogenic phenomena, †Current address: National Marine Fisheries Service, 203 13th Avenue South, St. Petersburg, FL 33701 coupled with intense commercial and recreational fisheries, ‡Current address: Department of Biology and Marine Biology and the have greatly reduced the number of dense aggregations and Center for Marine Science, University of North Carolina-Wilmington, overall population sizes of bay scallops in both the Atlantic and 5600 Marvin K. Moss Lane, Wilmington, NC 28409 Gulf during the past few decades (Peterson & Summerson 1992, DOI: 10.2983/035.030.0302 Tettelbach & Wenczel 1993, Arnold et al. 1998). 583 584 BERT ET AL. pleted wild populations or suitable unpopulated areas with aquacultured young individuals or placing aquacultured mature individuals in cages in the ocean and allowing them to spawn) (Tettelbach & Wenczel 1993, Peterson et al. 1996, Arnold et al. 2005, Tettelbach 2009, Tettelbach et al. 2010). The potential genetic impacts of both the depletions and the restorations are numerous, varied, and potentially threatening to the species’ ability to survive over evolutionary time (Bert et al. 2007). Here, we address these three conundrums. We evaluate and compare the population genetic structures of bay scallops from Atlantic and Florida Gulf waters using allozyme loci and a segment of the mitochondrial DNA (mtDNA) molecule. Col- lectively considering the information provided by analysis of both nuclear and mitochondrial genes often provides a more complete picture of population genetic structure and the mechanisms driving that structure (Edmands et al. 1996, Rigaa et al. 1997, Shaklee & Bentzen 1998, Busack & Lawson 2008). Together, the allozyme data and mtDNA data enabled us to provide a fresh interpretation of the taxonomic composition of bay scallops in the Atlantic area of sympatry between A. i. irradians and A. i. concentricus, and to probe the phylogeographic history of the genetic differentiation between the two subspecies. We then draw upon both data sets to resolve the taxonomic ambiguity of A. i. taylorae and deepen our understanding of its population history. Last, we discuss the potential influence of the common fishery management practice of hatchery-based stock enhancement on Atlantic bay scallop population genetic struc- ture and speculate about its impacts on Atlantic bay scallop populations. Our results are particularly relevant because they are based on genetic data collected prior to stock enhancement efforts for Florida Gulf bay scallops, and much stock enhance- ment has continued in Atlantic bay scallop populations since our samples were collected. Future studies using the same loci and collecting sites could reveal the effects of the numerous, sub- sequent stock enhancement programs on the population genetic structures of both Florida Gulf and Atlantic bay scallops. METHODS AND MATERIALS Field and Laboratory From 1995 through 1998, we obtained collections of bay scallops from 15 locations in eastern U.S. nearshore waters (Table 1, Fig. 1B). Whole wild bay scallops were collected from Atlantic locations during summers and were shipped alive to us by colleagues. Scuba divers collected adult bay scallops from 12 locations in the Florida Gulf during annual surveys. All bay Figure 1. Bay scallop, Argopecten irradians, ranges and sampling scallops were dissected, and samples of adductor muscle, gills, locations. (A) Subspecies distributions. Asterisks after abbreviations and digestive gland were excised, wrapped, immediately frozen denote collecting locations. FL, Florida; GA, Georgia; LA, Louisiana; in liquid nitrogen, and stored at –80°C. MA, Massachusetts; MD, Maryland; ME, Maine; NJ, New Jersey: NY, New York; NC, North Carolina; SC, South Carolina; TX, Texas; VA, For allozyme electrophoresis, small pieces of the three tissue Virginia. (B) Florida collecting locations ($ subpopulations; Bert et al. in types were combined and homogenized in 0.1 M Tris–EDTA, prep.) grouped into the 4 populations used in this report. pH 7.0; the supernatant was used as the enzyme source. Hori- zontal starch gel